[Show abstract][Hide abstract] ABSTRACT: Carrier injection from Au
electrodes to organic thin-film active layers can be greatly improved for both electrons and holes by nano-structural surface control of organic semiconducting thin films using long-chain aliphatic molecules on a SiO2 gate insulator. In this paper, we demonstrate a stark contrast for a 2,5-bis(4-biphenylyl)bithiophene (BP2T) active semiconducting layer grown on a modified SiO2
dielectric gate insulator between two different modifications of tetratetracontane and poly(methyl methacrylate) thin films. Important evidence that the field effect transistor
(FET) characteristics are independent of electrode metals with different work functions is given by the observation of a conversion of the metal-semiconductor contact from the Schottky limit to the Bardeen limit. An air-stable light emitting FET with an Au
electrode is demonstrated.
[Show abstract][Hide abstract] ABSTRACT: High-quality polycrystalline polyacenes (PLAs) of anthracene (AN), tetracene (TN), and pentacene (PN) doped with potassium in the 1:1 and 1:2 stoichiometries are prepared and their physical properties are systematically studied by magnetic measurements. The magnetic susceptibilities as a function of temperature for Kx(PLAs) are interpreted in the framework of a band-filling model. Clear evidence for the occurrence of charge transfer from K to the organic semiconductors is provided. K1(AN) prepared by a solid-state diffusion process exhibits a pronounced magnetic hump at 140 K, which is associated with antiferromagnetic interactions ascribed to the on-site Coulomb repulsion in a Mott insulating state, whereas both K1(TN) and K1(PN) show a conventional Curie–Weiss behavior at low temperatures. We successfully demonstrate the tuning of the electronic states of PLA-type semiconductors in bulk. Comments are given about the possibility of metallic states in potassium-doped PLAs, which have recently been reported for other polycyclic aromatic hydrocarbons.
No preview · Article · Aug 2014 · Berichte der deutschen chemischen Gesellschaft
[Show abstract][Hide abstract] ABSTRACT: A systematic study on the anharmonicity of phonons is made for single-crystal type-I clathrates: n-type Ba8Ga16Ge30 (n-BGG), p-type Ba8Ga16Ge30 (p-BGG), n-type Sr8Ga16Ge30 (n-SGG), n-type K8Ga16Sn30 (n-KGSn), and n-type Ba8Ga16Sn30 (n-BGSn), based on their heat capacity Cp at low temperatures (T) down to 360 mK. The low-T linear terms obsγT of Cp, including the tunneling term of the atoms accommodated in the host cages (γphT=αT) and the Sommerfeld itinerant-electron term (γeT), are successfully separated through careful measurements of single crystals with various carrier concentrations. The values of the minimum density of anharmonic potentials are deduced from α to be 0.12±0.24 for n-KGSn, 0.47±0.24 for n-BGG, 1.9±0.8 for p-BGG, 6.0±0.9 for n-SGG, and 10.9±0.7 for n-BGSn in units of ×1015 cm−3. The effective mass (m*/m0) is determined from γe to be 1.01±0.25 for n-BGG, 1.20±0.19 for p-BGG, 1.68±0.24 for n-SGG, 1.86±0.54 for n-KGSn, and 2.05±0.48 for n-BGSn in units of free electron mass m0, and the electron-phonon interaction strength can be evaluated from these values. It is shown that both the thermal conductivity (κ) and the electron-phonon interaction strength (λ) agree well with the α parameters deduced from Cp. The differences in κ known between n- and p-BGGs are ascribed to the influence of defects at the crystallographic 6c sites, which are clearly indicated by magnetic susceptibility measurements. This is very different from the situation in the other clathrates.
Full-text · Article · Jun 2014 · Physical Review B
[Show abstract][Hide abstract] ABSTRACT: Field-effect transistors that employ an electrolyte in place of a gate dielectric layer can accumulate ultrahigh-density carriers not only on a well-defined channel (e.g., a two-dimensional surface) but also on any irregularly shaped channel material. Here, on thin films of 95% pure metallic and semiconducting single-walled carbon nanotubes (SWNTs), the Fermi level is continuously tuned over a very wide range, while their electronic transport and absorption spectra are simultaneously monitored. It is found that the conductivity of not only the semiconducting but also the metallic SWNT thin films steeply changes when the Fermi level reaches the edges of one-dimensional subbands and that the conductivity is almost proportional to the number of subbands crossing the Fermi level, thereby exhibiting a one-dimensional nature of transport even in a tangled network structure and at room temperature.
No preview · Article · Jun 2014 · Advanced Functional Materials
[Show abstract][Hide abstract] ABSTRACT: High quality bulk samples of anthracene (AN) doped with potassium (K) in 1 : 1 and 2 : 1 stoichiometries were successfully prepared by a method involving a room temperature solid-state mechanical diffusion process prior to intercalation reactions during heat treatment, and their physical properties were studied using both magnetic and optical measurements. The transfer of almost one electron from K to AN in K1(AN) was confirmed by SQUID and ESR measurements. A pronounced magnetic hump centered at 150 K associated with antiferromagnetic interactions was observed, which can most likely be interpreted in terms of on-site Coulomb repulsions of the Mott insulating states. Optical spectra of K1(AN) clearly showed the insulating states, as well as the electron occupation of the LUMO-derived band of AN. Our results demonstrated tuning of the ground state of a typical bulk hydrocarbon by alkali metal intercalation.
No preview · Article · May 2014 · Dalton Transactions
[Show abstract][Hide abstract] ABSTRACT: Electric field tuning of superconductivity has been a long-standing issue in solid state physics since the invention of the field-effect transistor (FET) in 1960. Owing to limited available carrier density in conventional FET devices, electric-field-induced superconductivity was believed to be possible in principle but impossible in practice. However, in the past several years, this limitation has been overcome by the introduction of an electrochemical concept, and electric-field-induced superconductivity has been realized. In the electric double layer (EDL) formed at the electrochemical interfaces, an extremely high electric field is generated and hence high-density charge carriers sufficient to induce superconductivity exist and are collectively used as a charge accumulation device known as an EDL capacitor. Field-induced superconductivity has been used to establish the relationship between T-c and carrier density and can now be used to search for new superconductors. Here, we review electric-field-induced superconductivity using an FET device, with a particular focus on the latest advances in EDL transistors.
Full-text · Article · Mar 2014 · Journal of the Physical Society of Japan
[Show abstract][Hide abstract] ABSTRACT: As a methodology for controlling the carrier transport of topological insulators (TIs), a flexible tuning in carrier number on the surface states (SSs) of three-dimensional TIs by surface modifications using organic molecules is described. The principle of the carrier tuning and its type conversion of TIs presented in this research are based on the charge transfer of holes or electrons at the TI–organic molecule interface. When 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) as an electron acceptor or tetracyanoquinodimethane (TCNQ) as a donor is employed for n- and p-Bi2–xSbxTe3–ySey (BSTS) single crystals, successful carrier conversion from n- to p-type and its reverse mode is demonstrated depending on the electron affinities of the molecules. The present method provides a nondestructive and efficient method for local tuning in carrier density of TIs and is useful for future applications.
Full-text · Article · Feb 2014 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: Optoelectronic devices based on layered materials like graphene have resulted in significant interest due to their unique properties and potential technological applications. The electric and optoelectronic properties of nano GaTe flakes as layered materials are described in this article. The transistor fabricated from multilayer GaTe shows a p-type action with a hole mobility of about 0.2 cm2V-1s-1. The gate transistor exhibits a high photoresponsivity of 104 A/W, which is greatly better than that of graphene, MoS2 and other layered compounds. Meanwhile, the response speed of 6 ms is also very fast. Both the high photoresponsivity and the fast response time described in the present study strongly suggest that multilayer GaTe is a promising candidate for future optoelectronic and photosensitive device applications.
[Show abstract][Hide abstract] ABSTRACT: Taking advantages of broad tunability of carrier density in electric-double-layer transistors (EDLTs) with ionic-liquid gating, we demonstrate evidence of parallel conduction from both p-type bulk and n-type surface in Mg-doped InN EDLTs by comparing their transport properties, especially Hall effect, with those in non-doped InN. Large anomalous oscillation in Hall coefficients with decreasing gate bias was observed in Mg-doped samples, which can be well simulated by two-carrier model. Our results provide the proof for the p-type bulk conduction in Mg-doped InN by showing its prominent effects on electrical transport.
Full-text · Article · Dec 2013 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: Since the discovery of room temperature ferromagnetism in (Ti,Co)O2, the
mechanism has been under discussion for a decade. Particularly, the central
concern has been whether or not the ferromagnetic exchange interaction is
mediated by charge carriers like (Ga,Mn)As. Recent two studies on the control
of ferromagnetism in anatase (Ti,Co)O2 at room temperature via electric field
effect [Y. Yamada et al., Science 332, 1065 (2011)] and chemical doping [Y.
Yamada et al., Appl. Phys. Lett. 99, 242502 (2011)] indicate a principal role
of electrons in the carrier-mediated exchange interaction. In this article, the
authors review fundamental properties of anatase (Ti,Co)O2 and discuss the
carrier mediated ferromagnetism.
[Show abstract][Hide abstract] ABSTRACT: Transition-metal dichalcogenides such as WSe2 and MoS2 have electronic band structures that are ideal for hosting many exotic spin–orbit phenomena. Here we investigate the possibility to generate and modulate a giant Zeeman-type spin polarization in WSe2 under an external electric field. By tuning the perpendicular electric field applied to the WSe2 channel with an electric-double-layer transistor, we observe a systematic crossover from weak localization to weak anti-localization in magnetotransport. Our optical reflection measurements also reveal an electrically tunable exciton splitting. Using first-principles calculations, we propose that these are probably due to the emergence of a merely out-of-plane and momentum-independent spin splitting at and in the vicinity of the vertices of the WSe2 Brillouin zone under electric field. The non-magnetic approach for creating such an intriguing spin splitting keeps the system time-reversally invariant, thereby suggesting a new method for manipulating the spin degrees of freedom of electrons.
[Show abstract][Hide abstract] ABSTRACT: Resistivity, Hall resistivity, thermopower, thermal conductivity, and magnetization are reported for polycrystalline Ba8Ni5Ge41. Ba8Ni5Ge41 is diamagnetic with susceptibility χ
dia = (−2.4 to −2.82) × 10−7 emu/g. Semiconductor-like behavior was observed for the resistivity. The thermopower shows positive values for a wide temperature range. The Hall resistivity indicates the dominance of electrons, suggesting the existence of multiband conductance. At room temperature, the thermal conductivity is 1.78(5) W/K m. The highest ZT of Ba8Ni5Ge41 is 0.0016 at about 278 K.
No preview · Article · Jul 2013 · Journal of Electronic Materials
[Show abstract][Hide abstract] ABSTRACT: We report the synthesis and characterization of two new furan-based biphenyl end-capped oligomers, 2-([1,1′-biphenyl]-4-yl)-5-(5-([1,1′-biphenyl]-4-yl)thiophen-2-yl)furan (BPFT) and 5,5′-di([1,1′-biphenyl]-4-yl)-2,2′-bifuran (BP2F) as candidate semiconductors for organic light-emitting field effect transistors (OLETs). Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) showed the high thermostability of these furan-based semiconductors. X-Ray crystallography of single crystals grown by physical vapor transfer (PVT) method revealed a complicated herringbone packing of BPFT stacking with unusual flat and bent structures, which is different from that of BP2F and the bithiophene-based analogue 5,5′-di([1,1′-biphenyl]-4-yl)-2,2′-bithiophene (BP2T). BPFT single crystal showed a higher absolute quantum yield (51%) compared to that of BP2F and BP2T. Density Functional Theory (DFT) calculations showed that the different excitation energies between flat and bent structures led to the asymmetric transition dipoles in dark state of BPFT H-aggregates, which explains the highest PLQY of BPFT single crystal. Single crystal FET based on BPFT showed an ambipolar characteristic with high hole and electron mobilities, while single crystal FET based on BP2F exhibited p-type characteristic with a high hole mobility. Light emission was observed from the single-crystal FET based on BPFT.
[Show abstract][Hide abstract] ABSTRACT: Electrically manipulating electron spins based on Rashba spin-orbit coupling (SOC) is a key pathway for applications of spintronics and spin-based quantum computation. Two-dimensional electron systems (2DES) offer a particularly important SOC platform, where spin polarization can be tuned with an electric field perpendicular to the 2DES. Here, by measuring the tunable circular photogalvanic effect (CPGE), we present a room-temperature electric-field-modulated spin splitting of surface electrons on InN epitaxial thin films which is a good candidate to realize spin injection. The surface band bending and resulting CPGE current are successfully modulated by ionic liquid gating within an electric double-layer transistor configuration. The clear gate voltage dependence of CPGE current indicates that the spin splitting of the surface electron accumulation layer is effectively tuned, providing a way to modulate the injected spin polarization in potential spintronic devices.
[Show abstract][Hide abstract] ABSTRACT: We theoretically analyze the optoelectronic properties of single crystals of 2,5-bis(4-biphenylyl) bithiophene (BP2T) and 2-(4-biphenyl)-5-[5-(4-biphenyl)-2-thienyl] furan (BPFT) molecules, aiming to provide a guiding principle for the material design of organic light-emitting transistors. The X-ray structure analysis and the density functional theory (DFT) calculations indicate that half of the BPFT molecules bend the π-conjugation plane in the crystal. The Marcus theory parametrized by the DFT calculations indicates anisotropic charge mobilities. The emission spectra of the BP2T and BPFT crystals are analyzed by the time-dependent DFT calculations in conjunction with the Frenkel exciton model and the vibronic coupling analysis. We revealed that the high photoluminescence efficiency of the BPFT crystal originates from the symmetry breaking of the H-aggregate, where the transition dipole of the dark state does not cancel out.
No preview · Article · Apr 2013 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: Electric field induced ferromagnetism at room temperature in cobalt-doped titanium dioxide was demonstrated by means of electric double layer transistor. This result represents that a carrier-mediated exchange coupling plays a principal role in the high temperature ferromagnetism in this compound. Accordingly, this compound is a promising material for room temperature semiconductor spintronics.
[Show abstract][Hide abstract] ABSTRACT: Zintl phase clathrates, which are featured by the cage framework with
guest atoms accommodated inside, are considered as good candidates of
thermoelectric materials mainly due to the low thermal conductivity
caused by large scattering of the acoustic phonons via the rattling
phonons arising from the guest motions [1,2]. The fact has been known so
far that, in clathrate Sr8Ga16Ge30
showing off-centered displacement of encapsulated elements, thermal
conductivity is suppressed even stronger via the scattering of acoustic
phonons by anharmonic rattling phonons. Consequently, further detailed
understanding on the anharmonic potentials realized in clathrates is
important. In this meeting, we will present our recent studies on low
temperature specific heat of type I
K8Ga8Sn38 in addition to those of
Sr8Ga16Ge30 reported previously .
The discussion will mainly focus on the separation of the apparent
linear temperature dependent terms of anharmonic rattling phonons from
those of conduction electrons. The electron phonon interaction strength
and the tunneling density of anharmonic potentials will be described on
a basis of the analyses.  J. Tang, et al., Phys. Rev. Lett., 105,
176402 (2010).  J.-T. Xu, et al., Phys. Rev. B, 82, 085206 (2010).
[Show abstract][Hide abstract] ABSTRACT: Intercalated compounds of polycyclic aromatic hydrocarbons have been
drawing much attention from the view point of new type of organic
superconductors. The mechanism of superconductivity in these materials
is still unclear, and therefore the true ground states with various
carrier concentrations must be understood. The antiferromagnetic ground
states were reported particularly on K-doped pentacene, a typical
polyacene. In the present study, we focus on the synthesis and the
magnetic properties of K-intercalated polyacenes, such as anthracene,
tetracene, and pentacene. The improved synthetic method based on the
conventional solid state reaction was employed to obtain high quality
bulk samples. The X-ray powder diffraction profiles of doped samples
showed new stable phases. Interestingly, a pronounced hump at 150 K was
observed in the temperature dependence of magnetic susceptibility of
K1anthracene. In ESR measurements the linewidth of the
signals decreased significantly with a decrease in temperature below 150
K and no Pauli magnetic contribution was detected. These results clearly
indicate that charge transfer occurs but the most stable ground state is
still insulating via antiferromagnetic interactions. Further discussion
will be made among these K-intercalated polyacenes.
[Show abstract][Hide abstract] ABSTRACT: A topological insulator is a material that behaves as an insulator as a
bulk state, while permitting metallicity on its Dirac cone surface
state. One of the most serious issues of recent researches in this
field, however, has been the fact that the Fermi levels in many TIs
actually fall in either the conduction or valence band due to the
naturally occurring defects and must be controlled by further doping. We
report here that the major electron carriers on the SS of a
single crystal can be converted to the hole carriers via interface
strong electron affinity. The evidence can be elucidated using a
detailed three-carrier model. The results apparently demonstrate that
the charge transfer at the TI/organic-molecule interface is very
efficient in order to control the carrier density of TIs, particularly
on the SS. Our present results will be very important for studying the
fundamental aspects of TIs as well as their future device applications.
[Show abstract][Hide abstract] ABSTRACT: We here report the result of in situ magnetization measurements of
electrochemical cells at low temperatures. Upon applying voltages
between the electrodes of the electrochemical cells, we observed
shielding diamagnetic signals from several materials, indicating
superconducting transitions. The superconducting states can be induced
both electrochemically and electrostatically with appropriate
combination of counter electrode materials and electrolytes. The present
technique may become a powerful method for searching novel
Full-text · Article · Dec 2012 · Journal of Physics Conference Series